"ZenQueue is an incredibly simple message queueing system. It was designed as an experiment, but the author thought it was pretty neat and powerful and decided to release it to the world at large. The latest release of this library includes faster and more stable client and server implementations, in addition to both synchronous and asynchronous implementations of the client and queue. The server, however, only exists in asynchronous mode."

"Gearman provides a generic framework to farm out work to other machines or dispatch function calls to machines that are better suited to do the work. It allows you to do work in parallel, to load balance processing, and to call functions between languages. It can be used in a variety of applications, from high-availability web sites to the transport for database replication. In other words, it is the nervous system for how distributed processing communicates."

This is a toy in-memory (very much non-durable) queue, with a client and server in Python and a server in C++ -- the results of an evening's hacking with Facebook's Thrift protocol/RPC generator.

"PIMP Is Mostly Push". Heh. Joshua Schachter's ideas about an Atom push messaging system.

"a language-neutral, platform-neutral, extensible way of serializing structured data for use in communications protocols, data storage, and more."

open source messaging queue

A distributed, peer-to-peer, server-less communication & messaging solution for JAVA (JMS), C++ and .NET applications, integrates with JBoss, WebLogic & WebSphere. Offers guaranteed delivery, security and transactions. Supports TCP, SSL & HTTP protocols.

PHPMQ is an Open Source serverless messaging toolkit for PHP, giving the PHP developer ability to perform JMS operations such as sending and receiving messages on queues and topics while ensuring delivery and enabling persistent messaging.

"Stomp messaging server with file/dbm/memory/activerecord based FIFO queues, queue monitoring, and basic authentication."

"Spread is an open source toolkit that provides a high performance messaging service that is resilient to faults across local and wide area networks."